The present invention relates to an improvement of display quality of a liquid crystal display device. The invention particularly relates to prevention of a light leakage due to alignment disorder of liquid crystal caused by widening of a viewing angle.
As an operating mode of a liquid crystal display device, TN (Twisted Nematic) type is widely applied. The TN type has merits of easy gray scales display and a large aperture ratio, whereas has problems that when the viewing angle is changed, a change of transmittance is large and a viewing angle range is narrow. As one of methods of solving the problem that the viewing angle range of the TN type liquid crystal display device is narrow, a technique providing areas where strength of an electric field applied to the liquid crystal is different on a pixel is suggested. First, there will be explained briefly below the conventional technique. FIG. 16 is a plane view showing one pixel on an insulating substrate (hereinafter, referred to an array substrate) where a plurality of pixels, TFT (Thin Film Transistor) and the like are formed in the conventional liquid crystal display device. FIG. 17 is a sectional view taken along line E—E in FIG. 16.
In FIGS. 16 and 17, 1 is a first pixel electrode, 2 is a second pixel electrode, 3 is a gate line, 4 is a source line, 5 is a source electrode, 6 is a drain electrode, 7 is a semiconductor film, 9 is a storage capacitance line, 10 is a contact hole for connecting the first pixel electrode and the second pixel electrode, and 11 is a contact hole for connecting the second pixel electrode and the drain electrode. As shown in FIGS. 16 and 17, the pixel electrode of the conventional liquid crystal display device includes different two layers of the first pixel electrode 1 and the second pixel electrode 2. The second pixel electrode 2 is provided on a layer above an interlayer insulating film 15 provided on a layer above the first pixel electrode 1. Further, the second pixel electrode 2 is electrically connected with the drain electrode 6 by the contact hole 11 and is electrically connected with the first pixel electrode 1 by the contact hole 10. With such a structure, even in the case where the same voltage is applied to the first pixel electrode 1 and the second pixel electrode 2, areas where a voltage (electric field) to be applied to liquid crystal is different can be formed on one pixel. It becomes possible to widen the viewing angle range by varying the voltage to be applied to liquid crystal.
A conventional technique having different structure from the above one is disclosed in, for example, Japanese Patent No. 2809701. There will be explained below the conventional technique in the publication with reference to FIGS. 18 and 19. In FIGS. 18 and 19, the same reference numerals are given to the components which are the same as those in FIGS. 16 and 17, and a difference therebetween will be explained. FIG. 19 is a sectional view taken along line F—F in FIG. 18. In FIGS. 18 and 19, 12 is an insulating substrate, 16 is a liquid crystal, 17 is a counter substrate, 18 is a black matrix, 20 is a counter electrode, 21 is an alignment film on an array substrate side, 22 is an alignment film on the counter substrate side, 23 is a pixel electrode, 40 is an area which is not covered with the insulating film, 41 is a gate electrode, 42 is a thin film transistor, 43 is an insulating film, 44 is a low resistance semiconductor film, and 45 is an insulating film. As shown in FIGS. 18 and 19, the insulating film 45 on the pixel electrode 23 is removed in one pixel, and an area where the insulating film is formed and an area where the insulating film is not formed are provided on the pixel electrode. Thus, as is the case with FIGS. 16 and 17, the voltage to be applied to the liquid crystal can be varied in one pixel, and thus the viewing angle range can be widened.
There will be briefly explained below a mechanism for widening the viewing angle range. FIG. 20 shows a relationship between the voltage (V) to be applied to a liquid crystal and transmittance (T) in a normally white mode of the TN type liquid crystal display device. As shown in FIG. 20, in general there is a difference of approximately 1 to 2 V between a voltage at which transmittance starts to change (threshold voltage Vth) and a voltage at which a change of transmittance is almost finished (saturation voltage Vsat). In the liquid crystal display device, some voltage levels are provided between Vth and Vsat, thus gray scales display is executed. However, as shown in FIG. 20, in the TN type liquid crystal display device, in principle, when the viewing angle is changed, a V-T characteristic (voltage to be applied to the liquid crystal-transmittance characteristic) shifts and the transmittance changes greatly. As a result, the viewing angle range becomes narrow. However, when areas where the voltage to be applied to the liquid crystal varies are provided in one pixel, when the case having the first and second pixel electrodes in FIGS. 16 and 17 is exemplified, the V-T characteristics in the respective areas become as shown in FIG. 21(a) on the first pixel electrode and as shown in FIG. 21(b) on the second pixel electrode. An average of one pixel becomes a total sum of FIGS. 21(a) and 21(b) as shown in FIG. 21(c). For this reason, even if the viewing angle direction is changed, as shown in FIG. 22 a change of the transmittance in the case where the viewing angle is changed becomes small, and the viewing angle range can be widened.
As mentioned above, the areas where the voltage to be applied to the liquid crystal varies are provided in one pixel, thus the viewing angle can be widened. However, in the above-mentioned structure, since a voltage to be applied to the liquid crystal layer varies in a boundary portion between the areas where voltage to be applied to the liquid crystal varies in one pixel, when the case having the first and second pixel electrodes in FIGS. 16 and 17 is exemplified, equipotential surfaces such as Va, Vb and Vc shown in FIG. 23 are obtained in a vicinity of a pixel electrode opening portion, and an electric field component in a lateral direction is generated. In FIG. 23, the same reference numerals are given to the same components as those in FIGS. 16 through 19. Due to the lateral electric field in the boundary portion of the first and second pixel electrodes in FIG. 23, an alignment disorder of liquid crystal molecules positioned on this portion occurs. As a result, in the case, for example, where black display is executed in the liquid crystal display device in the normally white mode, a light leakage occurs on the boundary portion, and even when a sufficient voltage for black display is applied to the liquid crystal, the transmittance is not lowered sufficiently, and there arises a problem that the contrast is lowered.
In addition, in a structure which includes areas where the voltage to be applied to the liquid crystal varies in one pixel due to the two-layered pixel electrode composed of the first pixel electrode 1 and the second pixel electrode 2, as for the contact hole 10 for electrically connecting the first pixel electrode and the second pixel electrode, its level difference (level difference between the gate insulating film 14 and the interlayer insulating film 15) is large. Therefore, a light leakage which is caused by unsatisfactory alignment treatment due to rubbing occurs, and there also arises the problem that the contrast is lowered.
The present invention is devised in consideration of the above problem, in a structure which includes areas where voltage to be applied to a liquid crystal varies in one pixel, and it is an object of the present invention to provide a liquid crystal display device which a viewing angle is widened and contrast is high and fabricating method of the liquid crystal display device by preventing a light leakage from a boundary portion between the areas where the voltage to be applied to the liquid crystal varies, and reducing a light leakage on a contact hole provided for electrically connecting the first pixel electrode and the second pixel electrode in the case where the areas where the voltage to be applied to the liquid crystal varies are composed of two-layered pixel electrodes, namely, a first pixel electrode and a second pixel electrode.